In order to protect a transformer against an internal fault, a differential current type relay system has been utilized in the case of a generator and a power transmission line. The differential current type relay system is a method for deciding the internal faults when the differential current is greater than a predetermined value by obtaining the differential current between both terminals of the transformer. However, the differential current is largely caused by a large electric current through the primary coil, which is called an inrush. In this case a differential current can occurs other than internal faults.
Since the current of the primary coil contains a great amount of harmonic wave components at the inrush phenomenon, a second harmonic suppression differential current method, which uses the differential current as a driving current for driving the relay and a second harmonic component as a suppression current, is utilized in order to distinguish the internal fault and the inrush phenomena. In order to distinguish the internal fault and an over-exciting phenomenon, a fifth harmonic suppression differential current method utilizes a fifth harmonic component. However, since the size of the respective harmonic components can be changed depending on whether magnetic flux is residual or not, or depending on the size of the remanent magnetic flux as well as the second harmonic component can be changed even in an internal fault if the system's condition and material of the core of the transformer are changed, it is difficult to accurately tell the internal fault from the inrush. Moreover, since the method makes its determination in a frequency region in order to obtain the harmonic components, the time to accumulate data during one time period is required and so the rapid determination cannot be achieved.